Increasingly stringent legal provisions in respect of permissible pollutant emissions from motor vehicles powered by internal combustion engines make it necessary to minimize the pollutant emissions during operation of the internal combustion engine. This can be achieved on the one hand by reducing the pollutant emissions produced during combustion of the air/fuel mixture in the particular cylinder of the internal combustion engine. On the other hand, exhaust after-treatment systems are used in internal combustion engines to convert the pollutant emissions produced during the combustion process of the air/fuel mixture in the respective cylinders into harmless substances. For this purpose, catalytic converters are used which convert the carbon monoxide, hydrocarbons and nitrogen oxides into harness substances. Selectively controlling the generation of the pollutant emissions during combustion and converting the pollutants with a high degree of efficiency by means of a catalytic converter both require a very precisely adjusted air/fuel ratio in the particular cylinder.
In order to be able to meet current or even future legal requirements in respect of exhaust emissions, catalytic converters mounted close to the engine are being increasingly used. Because of the short mixing section from the internal combustion engine's outlet valves to the catalytic converter, these sometimes require very tight tolerances in the air/fuel ratio in the respective cylinders of the internal combustion engine, namely in the ratio to one another relative to an exhaust bank, compared to mounting the catalytic converter away from the engine, e.g. in an underfloor arrangement.
DE 199 03 721 C1 discloses a method for a multi-cylinder internal combustion engine for controlling, in a cylinder selective manner, an air/fuel mixture to be combusted, wherein the lambda values for different cylinders or cylinder groups are separately sensed and controlled. For this purpose a probe analysis unit is provided in which the exhaust probe signal is analyzed in a time resolved manner to determine a cylinder-selective lambda value for each cylinder of the internal combustion engine. Each cylinder is assigned an individual controller which is implemented as a PI or PID controller whose controlled variable is an individual cylinder lambda value and whose command variable is a individual cylinder lambda setpoint. The manipulated variable of the respective controller then influences the injection of fuel into the assigned cylinder.
The effectiveness of individual cylinder lambda control is critically dependent on how precisely the exhaust probe's measurement signal is correlated with the exhaust gas of the respective cylinder. During operation of the exhaust probe, its response may vary and therefore also the degree of precision of the assignment of the probe measurement signal to the exhaust gases of the particular cylinder.